1. Field of the Invention
The present invention relates to an optical disc to record data and a reproducing method of an optical disc and, more particularly, to a recording medium and its reproducing method in which the start position of a sector can be always detected even if recording is done by a modulating system, for instance, NRZI which does not have a pattern which is impossible on a modulating rule.
2. Description of the Prior Art
In an optical disc, one track is divided into a plurality of sectors and data is recorded/reproduced on a sector unit basis. There has been proposed an optical disc in which the sector is further divided into a plurality of segments and a servo byte is provided between the segments.
In such an optical disc, hitherto, a pattern which is impossible on a modulating rule is provided in, for instance, a header in the head segment of each sector. When a desired sector is accessed, the pattern which does not exist in the modulating system is detected first.
That is, in such a kind of conventional optical disc, as shown in FIG. 1, one sector is divided into a plurality of segments. A head segment 50 of one sector and two segments 51 and 52 following the segment 50 are set to headers H1, H2, and H3, respectively. Segments 53 subsequent to the head H3 are set to data areas.
A servo byte 54 is provided between the segments. As shown in FIG. 2, a pit 55 for clock reproduction and wobble pits 56A and 56B for tracking are arranged in the servo byte 54. In the example, pits 57A and 57B indicative of a Gray bode for track accessing are further arranged.
FIG. 3 shows a construction of the header H1 in the first segment. A servo byte 54A is arranged in the head of the header H1. A recording area 61 of a sector mark, a recording area 62 of the sector number, and a recording area 63 of an address are provided in the header H1.
As shown in FIG. 3, a pit 58 to form a unique distance which does not appear in the case of, for instance, the 4/11 modulation as disclosed in Japanese Patent Laid Open Publication No. Hei 2-130764 is provided in the sector mark recording area 61 in the header H1. That is, the pit 58 is arranged at a position which is away from the wobble pit 56B of a servo byte 54A by a distance 16. This distance forms a unique distance which does not appear in the case of, for example, the 4/11 modulation. Pits 59A to 59C are pits indicative of the sector mark.
In the case of accessing to a target sector, a pattern of the unique distance is detected first. Thus, a pattern comprising the pits 56B and 58 is detected. If this pattern can be detected, it is possible to detect that the segment is the head segment 50. Further, the pit 55 for clock reproduction is detected and the clocks can be reproduced.
The use of NRZI (Non Return to Zero Inverted) as a modulating method of an optical disc has been examined. This is because the NRZI has an advantage such that a window margin is long.
In the conventional optical disc, for instance, the 4/11 modulation is used. According to such a modulating method, a pattern which does not appear on a modulating rule exists. Therefore, as mentioned above, a pattern which does not exist in the modulating system is provided in the header H1 and when a desired sector is accessed, it is possible to execute a control such as to detect first the pattern which does not appear on the modulating rule.
On the other hand, in the NRZI modulating code, a pattern which does not appear on the modulating rule does not exist. Therefore, when a desired sector is accessed it is impossible to execute a control such as to detect the pattern which does not appear on the modulating rule.
In the case of a magnetooptical disc, if a servo byte is formed by an embossed pit as a physical pit of a different reflectance and a data area is formed by a pit of an orientation of the magnetization of a perpendicular magnetizing film, reproducing methods for the servo byte and the data area are different. Therefore, the reproduction signal of the servo byte and the reproduction signal of the data area can be discriminated.
However, in the case of the read only optical disc, since both of the servo byte and the data area are formed by embossed pits, it is difficult to discriminate the reproduction signal of the servo byte and the reproduction signal of the data area.